The invention relates to a method of forming isolation layers of a semiconductor memory device and, more particularly, to a method of forming isolation layers of a semiconductor memory device with improved electrical characteristics by reducing the content of impurities that may be generated when the isolation layers are formed.
A semiconductor memory device includes a number of memory cells for storing data and a number of transistors for transferring driving voltages. A flash memory device includes a number of strings in each of which memory cells are connected in series. An isolation layer is formed between the respective strings in order to insulate the strings electrically.
Meanwhile, as the degree of integration of semiconductor memory devices increases, the widths of the strings have been narrowed and, therefore, the width of the isolation layer has also been narrowed. As the width of the isolation layer is narrowed, processes of forming the isolation layer have become more difficult to carry out, as described below in detail by taking a flash memory device as an example.
In a process for forming an isolation later in a semiconductor memory device, a tunnel dielectric layer and a conductive layer for a floating gate are formed over a semiconductor substrate. Trenches are formed by sequentially patterning the conductive layer and the tunnel dielectric layer of an isolation region. In particular, as the degree of integration of a semiconductor device increases, the depth of the trench becomes greater than the width of the trench, thereby increasing the aspect ratio of the trench. If the aspect ratio of the trench is increased, voids can be generated within the trench because the bottom of the trench is not all gap-filled until the top of the trench is covered in the gap-fill process of the trench. Further, if material or a formation method with excellent step coverage is employed, materials formed on the sidewalls of the trenches face each other, resulting in formation of a seam.
An etchant can penetrate into the void or seam in a subsequent etch process, generating etch damage. The void or seam also causes degradation of the electrical characteristics of the semiconductor memory device.
To solve the problems, an isolation layer can be formed by gap-filling the bottom of the trench with a first dielectric layer (e.g., a fluid material such as SOD (Spin On Dielectric)) and further forming a second dielectric layer having a dense film quality on the top of the first dielectric layer. The second dielectric layer can be formed of, for example, an HDP (High Density Plasma) layer. Here, if a gap-fill process is not convenient even in the formation process of the second dielectric layer, a third dielectric layer (for example, an HDP layer) may be further formed after widening the top width of the trench through an etch process. In this case, damage can be generated due to the etch process. This is described in detail with reference to FIG. 1.
FIG. 1 is a photograph illustrating isolation layers of a conventional semiconductor memory device. Referring to FIG. 1, a tunnel dielectric layer 12, a conductive layer 14 for a floating gate 14, and an isolation mask pattern 16 are sequentially formed in an active region 10 of a semiconductor substrate. A first dielectric layer (not shown) and a second dielectric layer 18 are formed within a trench. If a wet etch process is performed using an etch process of widening the top width of the trench as described above, there may be a difference in the wet etch rate between regions A in which the film quality of the second dielectric layers 18 is formed irregularly. Alternatively, if a dry etch process is performed instead of the wet etch process, fluorine (F) included in the second dielectric layer 18 is combined with the tunnel dielectric layer 12, which may lower a subsequent program threshold voltage.